This invention relates to devices that transfer heat from a heat source to a heat sink by means of a closed internal evaporation-condensation cycle. Heat pipes have long been known in the art. A typical heat pipe utilizes a closed vessel, containing a wick and working fluid. As heat is applied to one end of the heat pipe called the evaporator, the liquid vaporizes, and as vapor pressure builds, vapor is driven to the cooling area, called the condenser, where the vapor condenses. The resulting liquid condensate then returns by capillary action or by gravity to the evaporator to be used to repeat the cycle. The resultant structure is characterized by high thermal conductance and very low temperature drop.
The interior of the heat pipe normally contains a wick extending throughout its entire length. However, certain designs that rely on a gravity liquid return system may require a wick only in the evaporator to uniformly distribute the liquid. The free space inside the structure is the vapor passage and must be kept clear for efficient flow of vapor from the evaporator to the condenser.
In many heat pipe applications it is advantageous to control the thermal impedance of the device. For example, the heat coming from a solar collector may be required in the winter but not in the summer. Therefore, in the summer, the controllably variable conductance heat pipe would be turned off. When the heating season arrives, energy from the solar collector will be needed and the variable conductance heat pipe can be turned on. Another application of a variable conductance heat pipe is in the utilization of energy escaping in the flue stack from a home furnace. The variable conductance heat pipe can conduct the heat (waste heat) to the sidewalk and remove ice in the winter. It is disadvantageous however, to allow the heat pipe to run continuously since this does remove heat from the flue gas exhaust and may allow flue gas cooling below the dew point resulting in condensate accumulation. The variable conductance heat pipe can be actuated only when necessary so that furnace damage due to condensates from the flue gas is minimized.